Problem: What number could replace $r$ below? $\dfrac{1}{2} = \dfrac{2}{r}$
Explanation: The fraction on the left represents 1 out of 2 slices of a rectangular pie. How many total slices would we need if we want the same amount of pie in 2 slices? We would need to cut the pie into 4 slices. $\dfrac{1}{2} = \dfrac{2}{4}$ and so the answer is $4$ Another way to get the answer is to multiply by $\dfrac{2}{2}$ $\dfrac{2}{2} = \dfrac{1}{1} = 1$ so really we are multiplying by 1. The final equation is: $\dfrac{1}{2} \times \dfrac{2}{2} = \dfrac{2}{4} $ so our answer is $4$.